When reinforced by long fibers such as glass or carbon fiber, these materials are incapable of deformation by elongation, unlike metallic materials. The long fibers effectively prevent elongation.
The matrix may for example comprise a thermoplastic resin of known type, such as one from amongst those known as PEI, PEEK, PES or PPS. With matrices of this type and at a temperature above the softening temperature of the matrix, the matrix can be deformed in response to an applied force and the reinforcements which are arranged in layers in the matrix may be subject to differential displacements.
In order for a sheet to retain its mechanical properties after deformation which may generate internal faults, it is necessary that the sheet is formed at a sufficiently high temperature and under a pressure which ensures suitable compactness.
A method is described in document DE 3727926 for forming an article of material from sheets of fiber-reinforced thermoplastic material. In the method described, the sheet to be formed is hot-deformed by using a forming mould. Deformation is performed via a separation membrane to which sufficient pressure is applied to deform the membrane. The separation membrane is a sheet of aluminum, synthetic material, or rubber.
An aluminum sheet can only be used once, so it is desirable to use a resilient membrane of an elastomer.
The teaching in the above-mentioned document is insufficient to provide an industrial apparatus capable of producing articles at a rapid rate.
The first problem is that of rapid heating and cooling of the article after forming. The thermal inertia of the forming equipment such as that described in DE-A-3727926 considerably extends the heating time, and more particularly the cooling time.